The present invention relates generally to balloon catheters used for angioplasty.
Angioplasty, an accepted and well known medical practice involves inserting a balloon catheter into the blood vessel of a patient, maneuvering and steering the catheter through the patient's vessels to the site of the lesion with the balloon in an un-inflated form. The un-inflated balloon portion of the catheter is located within the blood vessel such that it crosses the lesion or reduced area. Pressurized inflation fluid is metered to the inflatable balloon through a lumen formed in the catheter to thus dilate the restricted area. The inflation fluid is generally a liquid and is applied at relatively high pressures, usually in the area of six to twelve atmospheres. As the balloon is inflated it expands and forces open the previously closed area of the blood vessel. Balloons used in angioplasty procedures such as this are generally fabricated by molding and have predetermined design dimensions such as length, wall thickness and nominal diameter. Balloon catheters are also used in other systems of the body for example the prostate and the urethra. Balloon catheters come in a large range of sizes and must be suitably dimensioned for their intended use.
The term, low pressure diameter, as used herein with reference to the balloon catheter, means the diameter of the balloon when it is inflated to two (2) atmospheres.
The term, expanded diameter, as used herein with reference to the balloon catheter, means the diameter of balloon when it is inflated to six (6) to (12) atmospheres.
All angioplasty balloons have a minimum pressure at which they will burst called the minimum burst pressure. The physician is aware of the minimum burst pressure angioplasty balloons that he or she uses and usually avoids inflating a balloon to the point where it bursts. The physician is also aware that each kind and size of angioplasty balloon has its own expansion characteristics. This characteristic is usually expressed as a number which is the decimal portion of a millimeter that the balloon will expand when one additional atmosphere of pressure is applied. For example a 3 millimeter (diameter) balloon may expand 0.10 millimeters for each additional atmosphere of pressure that is applied. In this example at 10 additional atmospheres of pressure the balloon would have a diameter of 4.00 millimeters. This stretching characteristic is a factor of both the wall thickness and the material from which the balloon is molded. If the diameter of a balloon is measured during inflation, and the diameter is plotted, as one coordinate, against the inflation pressure as the other coordinate, the resulting curve is called the compliance curve for that particular balloon. If a balloon is made of a material that results in a relatively large increase in diameter when the balloon is inflated to its expanded diameter, such a balloon is said to be a High-Compliant balloon, or is said to be a balloon with a high compliance curve. FIG. 1A, is a graph showing a set of compliance curves for catheter balloons. In FIG. 1A the inflation pressure, measured in atmospheres, is plotted along the X-axis and the balloon diameter measured in millimeters is plotted along the Y-axis. In FIG. 1A the compliance curve having the greatest inclination is labeled High-Compliant. A High-Compliant balloon has a relatively large increase in diameter in response to an increase in inflation pressure. It should be noted that balloons defined herein as High-Compliant balloons are commonly referred to in the trade as, "Compliant balloons" or balloons made from compliant plastic material.
If a balloon is made of a material that results in a relatively small increase in diameter when the balloon is inflated to its expanded diameter, such a balloon is said to be a Non-Compliant balloon, a balloon made from non compliant plastic material or a balloon with a low compliance curve. Referring to FIG. 1A, the compliance curve having the least inclination is labeled Non-Compliant. A Non-Compliant balloon has a relatively small increase in diameter in response to an increase in inflation pressure. In FIG. 1A the third compliance curve is labeled Intermediate Compliant and represents a balloon having compliant characteristics between High and Non-Compliant balloons. It should be noted that although only three compliance curves are shown in FIG. 1A, balloons having compliant anywhere between the High-Compliant and the Non-Compliant curves are available. It should also be noted that all compliance curves shown in FIG. 1A are linear (straight lines).
High-Compliant balloons are made from relatively soft or flexible polymeric materials. Examples of these materials are thermoplastic polymers, thermoplastic elastomers, polyethylene (high density, low density, intermediate density, linear low density), various copolymers and blends of polyethylene, ionomers, polyesters, polyurethanes, polycarbonates, polyamides, polyvinyl chloride, acrylonitrile-butadiene-styrene copolymers, polyether-polyester copolymers, and polyether-polyamide copolymers. A suitable copolymer material, polyolefin material is available from E. I. DuPont de Nemours and Co. (Wilmington, Del.), under the tradename Surlyn.RTM. Ionomer.
Intermediate-Compliant balloons are made of polyethylene and nylon materials.
Non-Compliant balloons are made from relatively rigid or stiff polymeric materials. These materials are thermoplastic polymers and thermoset polymeric materials. Some examples of such materials are poly(ethylene terephthalate), polyimide, thermoplastic polyimide, polyamides, polyesters, polycarbonates, polyphenylene sulfides, polypropylene and rigid polyurethanes. Non-Complaint balloons made from poly(ethylene terephthalate) are commonly referred to as PET balloons.
The compliant characteristics of an angioplasty balloon affects how the physician uses the balloon catheter. A Non-Compliant balloon, will increase in diameter by a maximum of 5% of its nominal diameter in response to increasing the pressure to as much as twenty atmospheres. Sixteen atmospheres is safely below the burst pressure of such a Non-Compliant balloon. However, when inflated to its expanded diameter, a Non-Compliant balloon becomes very hard.
When a physician encounters a lesion that has become calcified and is very hard and rigid he may select a Non-Compliant balloon, that will become very hard and function to crack the rigid calcified lesion. Non-Compliant balloons have the advantage over Compliant balloons in that they can be used to dilate and crack hard lesions. Also if a Non-Compliant balloon is located in a vessel, across a restricted area of the vessel, and an end or both ends extend into non restricted areas of the vessel, the pressure in the balloon can be increased in the balloon sufficient to dilate or crack the restricted area without risking the possibility of damaging adjacent non restricted portions of the vessel. Non-Compliant balloons have the disadvantage that they are not effective if the normal vessel size lies between the size range of the available Non-Compliant balloons. Another disadvantage of Non-Compliant balloons is that if the lesion or restriction recoils after being dilated to its desired diameter, the Non-Compliant balloon cannot be used to dilate the lesion or restriction to a diameter greater than the previous dilation to thus overcome the recoil.
A High-Compliant balloon, will increase in diameter 15% to 40% in response to increasing the inflation pressure to a point safely below its burst pressure. The advantage of a High-Compliant balloon over a Non-Compliant balloon is that fewer models of High-Compliant balloons are required to fill a range of sizes. Non-Compliant balloons are typically available in size increments of 0.25 mm while High-Compliant balloons typically have size increments of 0.50 mm. Also an off-sized artery (i.e. 2.90 mm) will be difficult to dilate with a Non-Compliant balloon. Another advantage of a High-Compliant balloon over a Non-Compliant balloon is that if a restriction, after being dilated to its desired diameter, recoils when the balloon is deflated, the High-Compliant balloon can be re-inflated to a higher pressure thus dilating the restriction to a diameter greater than its desired diameter resulting in a satisfactory post recoil lumen diameter. This process can be repeated until the restriction retains its desired diameter after deflation of the balloon. High-Compliant balloons also have disadvantages, for example they can not be successfully used to dilate a hard lesion. Also if a High-Compliant balloon is located across a restriction and an end or both ends of the balloon extend into non restricted areas, when high pressure is applied to the balloon, the pressure may not be sufficient to crack or dilate the restrict area but will dilate the non restricted area to diameters greater than their normal diameter. In this situation damage can be done to the non restricted portions of the vessel.
The advantage of a hybrid compliant balloon is that advantages of both the Compliant and Non-Compliant balloons can be obtained in a single catheter that can be sized to the artery by varying the inflation pressure. In situations where the pressure required for a successful dilation is not extraordinarily high, a hybrid compliant balloon will give the physician more flexibility in matching the balloon to the artery size, resulting in a more controlled dilation.
In some situations a physician may desire a High-Compliant balloon that can initially expanded a significant amounts. If after the blood vessel adjacent to the restriction has been dilated to its natural size or at most 10% larger than its natural size, and the lesion has not yielded completely, it is not desirable that the balloon size be further increased due to the high rate of restenosis and dissection. In a situation such as this the physician may, after the restriction has not yielded sufficiently with the High-Compliant balloon, desire to remove the High-Compliant balloon and replace it with a Non-Compliant balloon. The Non-Compliant balloon that would be selected in this situation would have a nominal diameter approximately equal to the natural diameter of the open blood vessel, and its desired function would be to tightly compress the lesion into the wall of the blood vessel. It is desirable in this situation that the inflated balloon becomes very hard and rigid but not expand to a diameter that is greater than the natural diameter of the blood vessel. To accomplish this with currently available balloon catheters the initial High-Compliant balloon must be removed and replaced with a Non-Compliant balloon. This has the disadvantage that the patient is exposed to the trauma of removing and replacing a balloon catheter, the procedure time is lengthened and there is the expense of two balloon catheters. These disadvantages can be avoided by use of a balloon catheter having a hybrid compliant curve of the type disclosed in the preferred embodiment of this invention.
In another situation, with a hard calcified lesion, a physician may desire a Non-Compliant balloon which is inflated to whatever high pressure is required to yield or crack the lesion. If upon deflation of the Non-Compliant balloon the vessel retains its dilated diameter then the procedure has been successful and the catheter is removed. If however the vessel recoils, then the physician may desire to `over expand` the lesion site to a diameter greater than the normal diameter of the vessel. A Compliant balloon would be selected in this situation, and since the lesion has been cracked, the lesion area can now be dilated to a diameter greater than its normal diameter through the application of pressures that will not damage the adjacent non restricted vessel. To accomplish this with currently available balloon catheters the initial Non-Compliant balloon must be removed and replaced with the second Compliant balloon. This has the disadvantage that the patient is exposed to the trauma of removing and replacing a balloon catheter, the procedure time is lengthened and there is the expense of two balloon catheters. These disadvantages can be avoided by use of a balloon catheter having a hybrid compliant curve of the type disclosed in an alternate embodiment of this invention.
Compliance curves of angioplasty balloons, in their usable range are linear, that is essentially a straight line. As a result a physicians choice, in the past, has been to select a balloon having a linear compliance curve that best meets his needs. Physicians often encounter medical situations where an angioplasty balloon having a nonlinear compliance curve is called for but balloon catheters with the desired compliance curve have not been available. For example a physician may have a medical situation in which he desires a balloon that will initially increase in diameter by 20% and then become very rigid and hard with little further increase in diameter. Another example would be the situation where two lesions are encountered, one that can be treated with a High-Complaint balloon and the other that requires a Non-Compliant balloon. Still, another example would be the situation where the physician encounters a medical situation in which he desires to initially dilate a hard lesion to the normal diameter of the vessel but does not know whether the dilated vessel will recoil. Balloon catheters having hybrid or nonlinear compliance curves that could satisfy the requirements of these examples are possible as a result of this invention.
FIG. 2A is a graph in which the balloon diameter, in millimeters, is plotted along the Y-axis and the pressure in atmospheres is plotted along the X-axis. FIG. 2A, shows the compliance curves for two particular balloon catheters. In FIG. 2A the compliance curve having the greatest inclination is labeled High-Compliant and the compliance curve having the lesser inclination is labeled Non-Compliant. In accordance with the subject invention a two layered balloon is used, one of the balloons being a High-Compliant balloon and the other being a Non-Compliant balloon. Inflation fluid is metered through the inflation lumen to the inner balloon. In the preferred embodiment the High-Compliant balloon is inflates during the initial inflation phase, for example from its low pressure diameter to a diameter equal to its low pressure diameter times 1.2. During this initial inflation phase the relatively steep slope of the compliance curve labeled High-Compliant is followed. When the pressure is increased above the point at which the High-Compliant balloon has expanded to a diameter equal to 1.2 times its low pressure diameter, then the second inflation phase begins and the Non-Compliant balloon begins to expand, however it follows the relatively shallow slope of the compliance curve labeled Non-Compliant. The result is, as shown by the compliance curves shown in full lines in FIG. 2A. The full line compliance curve of FIG. 2A is a non linear compliance curve which is a hybrid of the compliance curves of the two balloons.
The prior art includes several patents that disclose double layered catheter balloons however the purposes for the double layer balloons in these prior art patents are different than the purpose for the double layer balloon arrangement of this invention. The prior art discloses the concept of independently inflatable, concentric balloons having different diameters. The smaller balloon is inflated first and if additional dilation is required the larger balloon is inflated. The purpose for other prior art double balloons is to provide for a small profile when the balloon is un-inflated, one balloon functioning as a holding chamber for the other or to facilitate the unfurling and inflation of the balloon when it is located in a lesion. Such prior art patents are U.S. Pat. Nos. 4,744,366; 4,649,914; 4,637,396; 4,608,984; 4,403,612; 4,338,942 and 4,327,736.
It is a primary objective of the present invention to provide a balloon catheter having a nonlinear compliance curve that has a particular use in a medical procedure.
Another objective of the present invention is to provide a balloon catheter having a two layered balloon in the inflation area that has a nonlinear compliance curve that has a particular use in medical practice.
Another objective of the present invention is to provide a dual layer balloon, one having a high compliance curve and the other having a low compliance curve, that together combine to provide a nonlinear compliance curve.